Some factors hindering the operation of conventional (“hard-switched”) PWM converters at higher switching frequencies are circuit parasitics, such as semiconductor junction capacitances, transformer leakage inductances, and rectifier reverse recovery. These parasitics can introduce additional switching losses and increase component stresses, and, consequently, limit the maximum frequency of operation of “hard-switched” converters. To operate converters at higher switching frequencies and, eventually, achieve higher power densities, it is helpful to eliminate, or at least reduce, the detrimental effects of parasitics while mitigating a degradation of conversion efficiency.
Generally, systems have attempted to eliminate these detrimental effects using one or both of two techniques: (1) resonant techniques (e.g., the introduction of permanent resonant tank circuits); and (2) constant-frequency PWM soft-switching techniques.
The common feature among the resonant techniques is the employment of a resonant tank circuit that is used to shape the current and voltage waveforms of the semiconductor switch(es) to create conditions for either zero-current turn-off, or zero-voltage turn-on. However, Zero-Current Switching (ZCS) and/or Zero-Voltage-Switching (ZVS) in resonant-type converters is often achieved at the expense of increased current and/or voltage stresses on the semiconductors in the system, as compared to the stresses in the corresponding “hard-switched” topologies. In addition, the majority of resonant topologies need to circulate a significant amount of energy to create ZCS or ZVS conditions, which increases conduction losses.
This strong trade-off between the switching-loss savings and increased conduction losses may result in a lower efficiency and/or larger size of a high-frequency resonant-type converter compared to its PWM counterpart operating at a lower frequency. This is often the case in applications with a wide input-voltage range. In addition, variable frequency of operation is often perceived as a disadvantage of resonant converters.